This invention relates to overhead projection devices and more specifically to a projector employing a liquid-crystal display (LCD) to project electronic images such as those of color television. Overhead projectors are in widespread use and are found in hundreds of thousands of classrooms and meeting areas. With the advent of computers, video recorders, laser discs, etc., there has been an increasing need to provide a convenient and effective way of producing a large-screen display of such information. LCD plates placed upon overhead projector stages are being used, but they are expensive due to their necessarily-large display areas, and tend to deteriorate from stage heat.
Other solutions have included separate projectors or television sets, but the combined cost is high and several units in a room are space-consuming and frequently inconvenient. That contrasts with the widely-accepted permanent positioning of an overhead projector and its screen. Accordingly, a primary object of the present invention is to extend the capabilities of an overhead projector to handle TV and computer imaging at relatively low cost with high quality. A small LCD device used in the invention is placed above and away from the projector stage to minimize heat problems. The overhead optics are unchanged but are augmented by an additional low cost element to provide very high optical efficiency.
In one embodiment of the invention all components are provided in a single small removable package. The package contains all necessary optical elements, an LCD device, heat-controlling components, and an electrical interface. The projected picture can be large and bright, and typically has the width of a standard overhead-projection image. The total package weight is less than ten pounds, and a maximum dimension is in the order of ten inches. The invention produces negligible heat and consumes merely four watts of power. In other embodiments the invention can be permanently or semi-permanently attached to an overhead projector with a greater number of shared features.
In a preferred separate-package embodiment the invention optics are resiliently mounted to provide automatic and simple coupling to the optics already provided in an overhead projector. The projector's focusing adjustment then can control the invention's optics, maintaining proper focus and magnification throughout an entire focusing range.
Another object of the invention is to exploit the efficient collimated-ray feature of an overhead projector. Its depth-of-field and image sharpness are largely due to the manner by which its light rays pass through a typical overhead transparency at nearly right angles to the transparency's surface and later enter the projection lens in a similar manner. Lens aberrations are reduced, and the transparency can be shifted away from a midfocus position a considerable distance before defocusing sets in. The present invention maintains that feature in an inexpensive manner by imposing a focusing lens only immediately prior to the first surface of the overhead projector's optical head. In a highly-light-efficient version, light collection and collimating elements are interposed between the LCD device and the stage. Even when the LCD device is far away from the stage, the TV image is sharp, bright, and easily focused without expensive lenses.
Still another object is to remove the LCD device from the relatively hot stage so that heat transmission can be controlled and cool operation achieved. The invention provides spectral optical filtering and other protective measures to prevent potentially harmful components of the overhead projector's radiation from reaching the LCD.